To date more than 99.9% of electricity generated worldwide is from some form of generator with rotational movement. Solar panels account for about 0.05%. Between 65 and 70% of the world industrial power and about 57% of all consumed power is used by electric motors. This relates to an estimated 16,000-plus terawatt-hours (TWh) annual consumption of electrical power worldwide. Due to this trend of consumption and efficiency improvement, conventional modern electrical generators and motors can operate in the 90 to 98% efficiency range near their rated revolutions per minute (RPM) and torque specifications. For this reason it is thought that the modern generator and motor industries are very mature and small incremental improvements can be made. However, while the narrow band of high efficiency rating in generators and motors is high, when these same generators and motors are operating outside the specified RPM and/or torque rating, the efficiencies dramatically decrease sometimes as low as 30 to 60%.
While most conventional generation systems use a continuous RPM and torque power source, renewable energies that are now emerging have much greater RPM and torque changes, as the power source is variable, untimely and most times unpredictable. As our capacity in conventional generation and distribution is reached, the need for generators in the renewable energies to be sensitive to this torque and still be efficient can be a very high priority. Likewise in the motor sector there exists a greater need for wider operating ranges with high efficiency for the industrial use and especially in the transportation sector as the demand for hybrid and “plug in” electric vehicles increases exponentially. An electrical motor's efficiency rarely stays constant, as the real world operating conditions require starts, stops and variable loads.
The modern day vehicle alternator converts some of the rotational power of the combustion engine into electrical power in order to operate the electronics and maintain battery charge. These alternators generally are 50 to 60% efficient. In 2007 there were about 806 million vehicles and today it is estimated to exceed a billion in operation. Almost 16% of manmade CO2 comes from these vehicles. Even a small amount of efficiency improvement in these alternators can make a dramatic improvement in fewer emissions and a considerable decrease in fuel consumption. This alternator efficiency loss is due primarily to air gap and inefficiencies in the rotor coil system (electromagnet). Permanent magnets in the rotor are not generally used in vehicular alternators due to the inability to regulate the output for variable loads efficiently.
Permanent magnet alternators (PMA) are used in small wind machines today. They typically have a high startup speed, as cogging of the rotor and the natural magnetic attraction of the stator tend to require a substantial minimum wind speed in order to overcome this limitation. They also lack the RPM range required to produce efficient power in the lower speed range as well as having a current limitation at very high wind speeds. They do not have the ability to regulate their output as the construction allows maximum power production at a given RPM. The stator selection limits the maximum current or voltage; it has a very limited efficiency range.
With medium to large wind systems, large AC generators are used and are converted to DC. Then power invertors invert the DC power signal to AC and distribute this current to the grid. This conversion comes with lost efficiency and heat production. This also limits the turbine startup speed and maximum output power. In large wind turbines, synchronous 3-phase generators can be used that usually have the rotor powered by the electrical grid in order to tie into the power grid frequency. While using the power inverter system to regulate the output power, they lose efficiency as well as limiting the turbine RPM range. Other renewable energy system generators such as tidal and wave generators have the same problems with efficiency loss due to limited RPM and torque ranges for the wide variations in RPM and torque range of these systems.
The use of permanent magnet motors in hybrid and “plug-in” electric vehicles has a very limited efficiency range as well. These motors like their PMA counterparts are limited by their construction in RPM, torque and current usage. They also have a problem with back EMF and extreme drag while in coast mode due to the permanent magnet passing continuously by the iron core of the stator.